Did you replace it with a new (packed from the factory) or another used Main Relay? If it was a used relay - it might have had the same problem as your original relay. Get your old relay (if you have it), and check this page -
http://www.markl.f9.co.uk/howto/elec...main-relay.htm
You will find info on how to check for "dry joints" on your relay. If you find any dry joints - most likely your other relay has the same problem. If it's not that, it would most likely be the Igniter Unit. Search for a thread titled "Sparkless & In The Snow" in this forum, and you'll find information on how to check if your igniter is good or not. If it's faulty - replace it.
When did you change spark plugs, spark plug wires, distributor cap and rotor?